Generally speaking, such jamming signals are received by the secondary lobes of a radar antenna at such a level that they considerably reduce the signal-to-noise ratio and completely peturb operation of the radar.
In order to reduce the interference thus produced on the useful signal, techniques have been developed known as secondary lobe cancelation (SLC). This countermeasure technique is descibed in outline in an article by M. A. Johnson and D. C. Stoner entitled "ECCM from the radar designer's viewpoint" published in the Microwave Journal, March 1978 at pages 59 and 60. This technique consists in adapting the radiation pattern of the receiver antenna as closely as possible to its environment in such a manner as to maximize the ratio of useful signal to the total interference. The adaption is done by using the reception paths of auxiliary antennas. The radiation patterns of the auxiliary antennas are combined with the pattern of the main antenna in question in such a manner as to obtain an overall pattern having nulls, or at least minimums, in the directions of the external jammers, while at the same time avoiding excessive amplification of the internal noise associated with the auxiliary antennas.
FIG. 1 summarizes the conventional circuit of a multijammer SLC system comprising a plurality of decorrelation loops.
A conventional SLC system is a "loop" system principally comprising a main antenna 1 and auxiliary antennas 2, 3, each of which is associated with a respective reception path 200, 300. Each reception path includes a loop comprising an amplifier 4, (40), an integrator 5, (50), a correlator 6, (60), and a control mixer 7, (70).
In such a prior art SLC system, each of the auxiliary signals b, (b') as received by an auxiliary antenna is subtracted in a summing circuit 8 from the main signal bO as received by the main antenna. The subtractions take place after the auxiliary signals have been multiplied by respective weighting coefficients W, (W') which are servo-controlled to the correlation existing between the corresponding auxiliary signal and the signal as used, in such a manner that the signal as used takes the form: bO-bW-b'W'. The noise is then minimum.
If a non-loop system is used, the optimum weighting coefficients may be calculated by a method which is equivalent to inverting the covariance matrix of the main signal by the auxiliary signals.
However, whichever algorithm is used, it can be shown that the choice of auxiliary antennas affects the speed at which the algorithm converges, the final improvement factor, the signal-to-jamming ratio, the bandwidth of the system, and the vulnerability of the system to additional jammers.
It thus appears that the auxiliary patterns, ie. the patterns of the auxiliary antennas, are important, and in the present invention, these patterns must be chosen carefully.
Generally, SLC auxiliary antennas, ie. antennas associated with prior art SLC systems, are not very directional, and they are often located around the periphery of the main antenna. Such a disposition has several drawbacks.
Since the auxiliary antennas are not very directional, and are sometimes practically omnidirectional, a single auxiliary antenna may cover several jammers in its pattern, thereby reducing the efficiency and the convergence speed of the weighting loops.
Since the gain of such an auxiliary antenna is low, a relatively high weighting coefficient must be applied to the signal it provides. This runs the risk of introducing a proportionately large friction of thermal noise from the associated receiver into the main path, thereby reducing the final improvement factor in the signal-to-jamming ratio. The improvement factor is the ratio of the signal-to-noise ratio with and without application of the noise power reducing method. In other words, the signal-to-noise ratio when the noise reducing method is applied divided by the signal-to-noise ratio when it is not applied.
The auxiliary pattern is broad and thus picks up parasitic echos known as clutter, thereby reducing the efficiency of the system.
The phase center of an auxiliary antenna is generally far from the phase center of the main antenna, and the associated weighting coefficient Wi is very sensitive to frequency.
For example, in the case of a frequency-agile radar, the weighting coefficient must change very quickly, thereby preventing the system from having a very large bandwidth.
Further, the overall pattern resulting from the combination of the main antenna pattern with the patterns of the poorly directional auxiliary antennas has sidelobes which are peturbed by the fact that the lobes of the auxiliary antennas pick up jammers which do not interfere with the main antenna when used on its own.
It can also be shown that there exist combinations of jammer directions and non:directive auxiliary antennas which do not converge to any solution at all. The set of quasi-point auxiliary sources together with their weighting coefficients constitute a pattern which is angularly periodic, while the sidelobes of the main antenna are not angularly periodic. Since the SLC system cancels one with the another, any arrangement which cancels in one direction is unlikely to cancel in other directions at one or more angular periods therefrom.
Preferred implementations of the present invention provide a method and apparatus for reducing the power of jamming signals received by the side lobes of a radar antenna which mitigate the drawbacks outlined above.